Silicone mold coating



This invention relates to mold coatings and is particularly directed to coatings for the interior surface of ingot molds prior to teeming steel.

The use of mold coatings on the surface of ingot molds prior to teeming or pouring is an established steel making practice. The necessity for coating or applying a mold dressing to the interior surface of the mold is caused by the fact that when the molten metal is poured or teemed from the ladle into the ingot mold it strikes the bottom of the mold and splashes onto the sides or interior wall surface. Such splashed metal clings to the mold Wall and fuses with the ingot surface when the molten metal rises in the mold, which causes a condition on the resulting metal ingots generally referred to as scabbing. When the splashed metal sticks to the mold wall it freezes or solidifies quickly and forms a surface oxide coating which does not dissolve entirely in the rising metal, hence the scab or defect. However, when the splashed metal falls back into the rising metal it remains molten or redissolves, and no defect is formed. Scabbing adversely affects the Surface of the ingots which require extensive conditioning to remove such defects.

In the past it has been the practice to provide the mold with a mold dressing or coating that will serve a a barrier between splashed metal and the mold surface so as to prevent the splashed metal from adhering to the mold Wall and thus prevent scabbing and the resultant detrimental effects to the ingot surface. Such conventional mold dressings are generally organic in nature; for example, pitch and tar are materials sometimes employed for such purposes. T he organic materials result in messy black fumes being expelled from the ingot mold during pouring. Such fumes are a health hazard and are highly undesirable from the standpoint of the personnel working in the vicinity of the teeming operation. In addition, and perhaps even more serious, is the fact that all known organic mold dressings are volatilized during the teeming operation and give off hydrogen gas which is readily dissolved by the molten metal. It is a well-known fact that hydrogen has the detrimental effect of embrittling steel products and therefore is a highly undesirable feature of the presently employed mold dressings. Other materials used as mold dressings, such as aluminum paint, do not effect the degree of non-wettability by the molten metals that is desired.

It has now been found that by employing silicones as a mold dressing, not only is a non-obnoxious, low hydrogen content vapor given off during teeming, but also a highly desirable silica refractory coating remains behind to further protect the mold surface.

An object of the present invention is to utilize a silicone mold dressing during the pouring of molten metal into ingot molds to prevent the formation of obnoxious fumes during teeming.

A further object and equally advantageous feature of the present invention is the method providing a silicone mold, dressing that, upon volatilization caused by teeming, emits a low hydrogen content gas.

Other objects and advantageous features will be obvious from the following specification:

The silicone coatings of the present invention volatilize in a similar manner to the organic compounds known to the art; however, the gas emitted by such mold dressings,

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such gas being considered to be essential in the function of the dressing in avoiding molten metal to mold wall contact, is not toxic, black or obnoxious as are the gases evolved from the wholly organic compounds. The silicones exhibit extraordinary non-wetting properties to molten steel in spite of the fact they decompose at temperatures far below that of the melting point of steel. Such an advantageous feature, alone, renders such new use of silicones to be novel; however, it has been found that of equal importance is the fact that such coatings serve to reduce the hydrogen pickup of the molten steel.

Although the use of silicon mold dressings is particularly advantageous to the steel industry, it is obvious that it may be applied to the metals industry in general. Silicone coatings have been found to be highly advantageous as mold coatings or crucible coatings for the copper crucibles employed in the vacuum arc melting process.

The term silicone as applied to the present invention and in the specification and claims is to be interpreted broadly, but in the manner of its usual and accepted definition in the organosilicone field. The term has the general meaning of designating compositions which are sometimes described as alkylated chains of alternate silicon atoms, sometimes containing side chains, rings and/or cross-links. Silicone is employed to designate those silicone compounds, somewhat analogous to the ketones, in which organic groups are combined with silicon and oxygen according to the general formulae, RSiO R SiO and R SiO R being a monovalent organic radical which may be of considerably diverse character and includes aryl as well as the alkyl radicals. Since silicon, unlike carbon, does not form a stable double bond with oxygen, the silicones in general are polymeric.

The molecular weight, mechanical and physical properties of the silicones are dependent on the degree of polymerization, the structural positioning of the polymer units and the nature of the organic radicals (represented by R above). The most common configurations are chain structures of the above basic units that either terminate in terminal groups, such as the hydroxyl group, or organic radicals such as the carboxyl group, and including the salts of the acids, or such chains may form cyclic structures (particularly those of 10 or less units). The chains or polymers of silicones may contain occasional cross links to adjacent and similar chains or such units may be cross linked in such a uniform manner as to form a structure similar to that of silica.

The silicones of cyclic and straight chain structure are usually liquids, those of cyclic structure having the lowest temperatures of vaporization and decomposition and straight chain polymers with occasional cross links form the bulk of the silicone rubbers or rubberlike resins, while the regular network of cross link type of structure represents the solid and usually hard resin type silicone.

Methods of preparing silicones, such as are described in the method of the present invention, are well known and are adequately described and taught in .the literature. Methods of preparing such materials and the properties of silicones are taught in many issued United States patents such as Patents Nos. 2,384,384 to McGregor et al., 2,382,032 to McGregor et al., 2,439,856 McGregor et al., 2,462,242 Web-b et al., 2,606,510 Collins, and 2,718,512 Warrick. Such textbooks as Organic Chemistry by Louis F. Fieser and Mary Fieser, third edition, D. C. Heath and Company, show the general structure of silicones.

Silicones of the type described above are available commerically under a wide variety of trade names, exhibiting a wide variety of properties obtained, not only by limitations on the degree of polymerization, but also by a wide variety of substitutions for the organic radicals represented above as R. R has been known to represent various substituted or unsubstituted aliphatic radicals, such as the methyl, ethyl, chloromethyl, hydroxy-meth'yl or isopropyl radicals and/or various substituted or unsubstituted aryl radicals, such as the phenyl, tolyl, xylol, chlorophenyl, dichlorophenyl or naphthyl radicals. The com merical product need not be a single polymer but may be a mixture of two or more of the above, the blend being 7 designed to meet specifically desired properties.

A few of the above-discussed silicones are water soluble, but many, particularly those of cyclic and terminal chain structure, are soluble only in benzene toluene and like products. Many of these polymers and blends of such materials are sold in the form of aqueous suspensions or colloid in which the water resistant silicones are agitated into a suspension in a fine particulate form while in the presence of an emulsifying agent.

In addition to the above compounds, the term silicone,- as employed in the present specification, includes such materials as the water-soluble salts of alkyl sil-iconic acids. Such salts may exist in the monomeric or polymeric form, but when employed as in the present invention are believed to be in the polymeric form containing RSiO units as described above. The preparation of water soluble alkali metal siliconates is described in United States Patent No. 2,507,200 of Elliott et al. An example of such material specifically employed as a mold dressing in the pouring of steel is a water solution of' about 30% sodium methyl siliconate.

For the purpose of the present invention, it is immateriai which of the above silicones are employed. It has been determined that any of the silicones commercially available as taught above may be employed. Although the presence of organic radicals (R above) does not seem to be adverse, it is preferable to employ silicones wherein such organic radicals do not exceed about 24 carbon atoms each.

The only consideration, outside of costs, one need give to the selection of silicone is its application to the mold surface. However, as stated above, nearly any of the silicones are available or may be made available either in solutions of benzene, toluene (or like substances), water or as aqueous dispersions or suspensions. Material in such a form may, of course, be easily applied to the mold surface by painting, spraying, etc. It is, of course, preferred to provide a substantially continuous coating of the silicone to protect the entire mold surface and to permit the coating to dry before teeming.

Examples of commercially available silicones available in fluid form which may be easily sprayed or painted onto the surface of the ingot mold are the silicones produced and sold by the General Electric Qompany, Silicone Products Department, Waterford, New York, and identified as follows:

SM-55 Silicone water emulsion. SM-dl Silicone water emulsion. SM62 Silicone water emulsion. 81509 Silicone water solution. 81644 Silicone fluid.

SF-96 Silicone fluid.

SI -92 Silicone fluid.

Viscasil Silicone fluid.

The above materials are emulsions or solutions of blends of the silicone materials referred to above. All contain from about to 40% silicones. It may be desirable to further dilute some of these materials before employing them as silicone mold dressings; however, all may be applied as purchased.

The following specific examples are given to illustrate the present invention and in no way limit the application to the exact embodiments set forth:

Example I AISI type 312 steel was cast or teemed into 34 ingot molds approximately 9" square x 42". Three molds were coated with General Electrics Silicone No. 81509 and the other 31 molds were smoked with rosin. General Electric Silicone No. 81509 is a water emulsion containing about 30% silicones. These silicones are of the straight chain dimethyl silicone polymer type of the general formulae (CH SiO with trimethyl silicone terminal groups. The degree of polymerization, of course, varies so that each molecule contains from about 10 to 12 of the (CH SiO units.

At the conclusion of casting and upon stripping the ingot, it was found that the three silicone coated ingot molds produced ingots which had a bright shiny surface showing a faint greenish cast, while the other 3i produced ingots with a dull surface. After being reduced to bi lets by conventional practice, the silicone treated ingots produced 5 square billets which were completely free of scabs, while the billets rolled from the untreated ingots showed considerable scabbiness, particularly on the bottom half of the billets. The silicone treated ingots were easily recognized during hot rolling due to complete freedom from scabs. Weight loss due to grinding amounted to only about 5.4% for the silicone treated ingots, while for the untreated ingots it was approximately 6.4%.

Example 11 Two consumable electrode vacuum melting crucibles (4" diameter), one constructed of aluminum and one of copper were coated with a silicone (General Electric Silicone 81509). Electrodes 0f A -286 alloy (an austenitic stainless steel) approximately 2.6" diameter x 21" were vacuum arc melted into the crucibles. No scabbing or surface defects were observable on the crucible wall after stripping the metal from the crucible. The ingot surface was exceptionally good.

Example Ill Laboratory experiments were conducted using an 8-inch diameter water-jacketed steel chamber coated with General Electric SR-53 Silicone resin. SR53 is a highly polymerized silicone used to impart water repellency to masonry or other materials. This silicone is available in a toluene solvent (10% solids). Twenty heats of molten steel (about 50 pounds each of various AISI types) were poured through this chamber while the pouring stream was purposely exploded by high velocity gas streams so that the walls of the chamber were sprayed with molten metal. No metal adhered to the walls of the chamber.

Example IV Molten AISI type 430 stainless steel was poured into a laboratory size ingot mold (4.5" square x 9" long) which had been coated with General Electric SC-50 Silicone. General Electric SC5O Silicone is a water solution containing about 30% sodium methyl siliconate. When the ingot mold was stripped from the ingot it was found that the ingot was scrab-free and exhibited a good surface.

The above examples are given to illustrate the method of the present invention and in no way limit the application or claims to he exact illustrative examples set forth.

We claim:

1. In the process of pouring molten metal into ingot molds the improvement of applying to the inside surface of said molds a substantially continuous coating of at least one material selected from the group consisting of polymers of at least one of the basic units having the general formulae RSiO R SiO, R SiO wherein R is an organic radical containing no more than about 24 carbon atoms and the salts of siliconic acids prior to said pouring.

2. In the process of pouring molten steel into ingot molds, the improvement of applying to the inside surface of said molds a substantially continuous coating of a polymer having the general formula R SiO, wherein R is an organic radical prior to said pouring.

3. In the process of pouring molten steel into ingot References Cited in the file of this patent UNITED STATES PATENTS 2,027,932 Ray Ian. 14, 1936 6 Barnes May 9, 1950 Hyde Oct. 30, 1951 Kennedy Mar. 17, 1953 Grant Sept. 18, 1956 Braley Oct. 29, 1957 FOREIGN PATENTS Great Britain Apr. 9, 1952 OTHER REFERENCES McGregor: Siiicones and Their Uses, McGraW-Hill Book Co., 1954, pages 108112. 

1. IN THE PROCESS OF POURING MOLTEN METAL INTO INGOT MOLDS THE IMPROVEMENT OF APPLYING TO THE INSIDE SURFACE OF SAID MOLDS A SUBSTANTIALLY CONTINUOUS COATING OF AT LEAST ONE MATERIAL SELECTED FROM THE GROUP CONSISTING OF POLYMERS OF AT LEAST ONE OF THE BASIC UNITS HAVING THE GENERAL FORMULAE RISIO15, R2SIO0.5, WHEREIN R IS AN ORGANIC RADICAL CONTAINING NO MORE THAN ABOUT 24 CARBON ATOMS AND THE SALTS OF SILICONIC ACIDS PRIOR TO SAID POURING. 